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An 80-year-old man has low back pain. An x-ray of the lower back and pelvis shows sclerotic changes in
the lower vertebrae and in focal areas throughout the pelvis. The radiologist's report states
that the sclerotic changes may represent osteoarthritis; however, metastatic prostate cancer
cannot be excluded. Which of the following is most cost-effective in the initial work-up of this
patient?
A. Bone marrow aspirate and biopsy
B. Digital rectal exam
C. Prostate-specific antigen
D. Radionuclide bone scan
E. Serum alkaline phosphatase
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The correct answer is B. Osteoarthritis is the most common rheumatologic disease, the
prevalence of which increases with age. It primarily involves weight-bearing joints, hence its
distribution in the lower vertebrae, pelvic bones, and proximal femur. Sclerotic bone,
representing reactive bone formation, develops as a reaction to injury and is responsible for
the slightly elevated serum alkaline phosphatase levels that normally occur in much of the
elderly population. If prostate cancer with osteoblastic (bone-forming) metastases to the
vertebral column and pelvis were present in this patient (stage D disease), a digital rectal
exam would be the most cost-effective initial step in the work-up. With advanced prostate
cancer, the gland would very likely be enlarged and hard ("stony").
A bone marrow aspirate and biopsy (choice A) is not usually part of the normal work-up of
possible metastatic prostate cancer and has no place in the evaluation of osteoarthritis.
A prostate-specific antigen (PSA; choice C) level should be ordered in this patient, but not as
the initial step in the work-up, since it does not distinguish hyperplasia from cancer and is
fairly expensive. In known cases of prostate cancer, the PSA is a measure of tumor burden and
is used to monitor recurrences when following patients who have been treated for prostate
cancer.
A radionuclide bone scan (choice D) is commonly used to rule out metastasis in patients with
prostate cancer. It is expensive and is not used as a screening test for prostate cancer.
The serum alkaline phosphatase (choice E) is typically elevated in metastatic prostate cancer
due to osteoblastic activity in the metastatic foci. However, an elevated serum alkaline
phosphatase is non-specific, since it may be slightly increased in osteoarthritis (reactive
bone formation) as well as in liver disease.
